Re: [Timber] bug (?) in ASYNC implementation

Ivan, Actually, this has been a point of contention for some time. I will not pretend to know ALL the arguments for the current solution (I hope Johan Nordlander can answer you), but here are some ideas: The "after"-construction is most commonly used (in the applications we've seen so far) to schedule actions past the current deadline. This case is not affected by this rule, and we do avoid any drift there. When an action is posted with "after" and the new baseline has already passed, then since the deadline for the posted action is always defined relative to its baseline, not adjusting the baseline to the current time would allow for a situation when not only the baseline, but also the deadline of the reaction has passed. It would make it impossible to verify that such as system meets its deadlines. Hope that makes some sense. Regards, Andrey On Oct 26, 2009, at 10:17, Ivan Tarasov wrote:

Andrey,

thanks for pointing out that this is documented. Unfortunately I don't understand the reasons for introducing that special case.

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Because the baseline offset 50 ms is measured relative the current baseline at each invocation (and not the actual time of the recursive call), the implementation does not suffer from accumulating drift. (from Johan Nordlander, Mark P. Jones, Magnus Carlsson, and Jan Jonsson, Programming with Time-Constrained Reactions. Technical report, Luleå University of Technology, 2005.)

Particularly, from my understanding of the following: this special case breaks the requirement of not causing the newly scheduled actions to drift.

Is there some other problem which this special case is intended to fix, which I don't recognize?

Regards, Ivan

2009/10/26 Andrey Kruglyak :

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From timber-lang.org >> Language summary >> Time constructs:

Time windows of reactions are assigned as follows:

The time window of a reaction to an external event has as baseline the time instant when the event occurs, and as deadline an idealised instant infinitely far into the future.

In particular, the start action of a program gets as baseline the time instant when program execution begins.

When a message without time constraints is sent (i.e., an plain action is called) from a method with current baseline bl and deadline dl, the reaction to the message inherits both bl and dl.

The rule in the previous item can be changed by explicit program constructs:

The expression after t act sets the effective baseline for act to the current baseline plus t.

The expression before t act sets the effective deadline for act to its effective baseline plust.

In both cases, act can be any expression of type Action, which includes the time-annotated constructs just introduced. Nested baseline offsets are composed using addition, while multiple deadline offsets are resolved by selecting the minimal value. The relative order of after and before constructs is not relevant.

Special case: if the baseline denoted by an after construct is an already passed time instant, the effective baseline of the reaction is rounded off to the actual time of the call.

Regards, Andrey On Oct 26, 2009, at 10:01, Ivan Tarasov wrote:

I'm trying to understand what constitutes an RTS and how a Timber program translated into C is actually executed. So, I'm reading the RTS implementations' code.

In rtsARM/rts.c, in ASYNC implementation, lines 439-442 we can see the following code: if (LESS(m->baseline, now)) { m->baseline = now; // debug("^"); } Same code is present in ASYNC implementation in rtsPOSIX/rts.c, lines 354-355

That means that if the absolute time value for the baseline for the action to be scheduled is in the past, we set the baseline to the current time. However, since all the time calculations are relative to the baseline, that would mean that all the following actions to be scheduled from the one being scheduled at the moment would "drift". Is this an intended behavior? I'm not sure that's the right decision.

Ivan

------------------------------------------------------------------------ Andrey Kruglyak PhD student http://www.andreykruglyak.com Dept. of Computer Science and Electrical Engineering Lulea University of Technology, Lulea, Sweden T +46 920 49 23 57 (office) T +46 706 16 76 03 (mobile) Office A2314 (A-huset)